File::Which finds the full or relative paths to executable programs on the
system. This is normally the function of "which" utility.
"which" is typically implemented as either a program or a built in
shell command. On some platforms, such as Microsoft Windows it is not provided
as part of the core operating system. This module provides a consistent API to
this functionality regardless of the underlying platform.

The focus of this module is correctness and portability. As a consequence
platforms where the current directory is implicitly part of the search path
such as Microsoft Windows will find executables in the current directory,
whereas on platforms such as UNIX where this is not the case executables in
the current directory will only be found if the current directory is
explicitly added to the path.

If you need a portable "which" on the command line in an environment
that does not provide it, install App::pwhich which provides a command line
interface to this API.

File::Which searches the directories of the user's "PATH" (the current
implementation uses File::Spec#path to determine the correct
"PATH"), looking for executable files having the name specified as a
parameter to "which". Under Win32 systems, which do not have a
notion of directly executable files, but uses special extensions such as
".exe" and ".bat" to identify them,
"File::Which" takes extra steps to assure that you will find the
correct file (so for example, you might be searching for "perl",
it'll try perl.exe, perl.bat, etc.)

Linux, *BSD and other UNIXes

There should not be any surprises here. The current directory will not be
searched unless it is explicitly added to the path.

Modern Windows (including NT, XP, Vista, 7, 8, 10 etc)

Windows NT has a special environment variable called "PATHEXT", which
is used by the shell to look for executable files. Usually, it will contain a
list in the form ".EXE;.BAT;.COM;.JS;.VBS" etc. If
"File::Which" finds such an environment variable, it parses the list
and uses it as the different extensions.

Cygwin

Cygwin provides a Unix-like environment for Microsoft Windows users. In most
ways it works like other Unix and Unix-like environments, but in a few key
aspects it works like Windows. As with other Unix environments, the current
directory is not included in the search unless it is explicitly included in
the search path. Like on Windows, files with ".EXE" or <.BAT>
extensions will be discovered even if they are not part of the query.
".COM" or extensions specified using the "PATHEXT"
environment variable will NOT be discovered without the fully qualified name,
however.

Windows 95, 98, ME, MS-DOS, OS/2

This set of operating systems don't have the "PATHEXT" variable, and
usually you will find executable files there with the extensions
".exe", ".bat" and (less likely) ".com".
"File::Which" uses this hardcoded list if it's running under Win32
but does not find a "PATHEXT" variable.

As of 2015 none of these platforms are tested frequently (or perhaps ever), but
the current maintainer is determined not to intentionally remove support for
older operating systems.

VMS

Same case as Windows 9x: uses ".exe" and ".com" (in that
order).

As of 2015 the current maintainer does not test on VMS, and is in fact not
certain it has ever been tested on VMS. If this platform is important to you
and you can help me verify and or support it on that platform please contact
me.

This module is fully supported back to Perl 5.8.1. It may work on 5.8.0. It
should work on Perl 5.6.x and I may even test on 5.6.2. I will accept patches
to maintain compatibility for such older Perls, but you may need to fix it on
5.6.x / 5.8.0 and send me a patch.

Not tested on VMS although there is platform specific code for those. Anyone who
haves a second would be very kind to send me a report of how it went.

Comes with a "can_run" function with slightly different
semantics that the traditional UNIX where. It will find executables in the
current directory, even though the current directory is not searched for
by default on Unix.

Devel::CheckBin

This module purports to "check that a command is available", but
does not provide any documentation on how you might use it.